1. Field of the Invention
The present invention relates generally to a front-and-back electrically conductive substrate and a method for manufacturing same, and more particularly, to a high-density front-and-back electrically conductive substrate capable of adapting to the high integration density, increasing post numbers and compactness of the LSIs required for electronic calculators and the like.
2. Description of Related Art
In recent years, the LSI packages and printed wiring boards have needed to acquire ever greater densities as the LSIs and other semiconductor components have become more highly integrated and equipped with a larger number of posts.
With the LSI packages, the LSI chip is mounted face down on a ceramic substrate or an organic substrate. Further, the ceramic substrate or organic substrate is mounted on a printed wiring board using balls of solder.
In order to obtain an electrical conduction between a first surface (that is, a front surface) on which the LSI chip is mounted and a second surface (a back surface) on which the solder balls are formed, through holes are formed in the substrate. A ceramic substrate or a resin substrate having such through-holes is called a front-and-back electrically conductive substrate.
The ceramic substrate is manufactured by punching holes in a green sheet material, such that the front surfaces of the through hole and the front surface of the green sheet are coated with copper plating, a plurality of green sheets are stacked one atop the other, pressed together, and then baked.
A resin substrate is produced by alternately stacking patterned copper sheets and prepreg sheets, pressing them together and then baking them. The baking temperature of the resin substrate is lower than the temperature at which ceramic substrates are baked. After the resin substrate is manufactured, through-holes are opened in the substrate and the surface is coated with copper plating.
It can be anticipated that integration densities will continue to increase in the future, and that as a consequence the diameter of the through holes formed in the substrate will decrease and the pitch of the through holes will decrease as a result.
However, a disadvantage of ceramic substrates is that there is a limit to how narrow the pitch of the through holes can be made, because the holes are formed by the mechanical process of punching the substrate and therefore the through-hole pitch cannot be made shorter than the distance by which the punch itself is fed onward.
In the case of organic substrates as well, the through holes are formed by the mechanical process of drilling holes in the substrate. As a result, organic substrates have the same disadvantage that attends ceramic substrates, that is, that the pitch of the through holes cannot be made shorter than the forward feed pitch of the drill. Moreover, although the drill used in the organic substrate can be made longer and thinner, doing so creates the possibility that the drill bit will break during operation, a possibility that increases as the depth, that is, the aspect, of the through holes increases. It should also be noted that forming the through holes in the ceramic substrate is a process performed one green sheet at a time and is not a high-aspect operation.
Additionally, there is the problem that the plating process that is performed after the through holes have been formed in the substrate will not completely cover the interior of the through holes, particularly as the aspect of the holes increases. As a result, some parts of the inner walls of the through holes may go unplated, thus degrading the reliability of the electrical conductivity.
In the case of the so-called multi chip module, or MCM, in which thin film technology is used to form thin film circuitry on the MCM, it is possible to set the LSI chip bump pitch minutely. However, either a ceramic substrate or a resin substrate as described above is used for the base substrate on which the thin film circuitry is formed, so the same problem of how to narrow the pitch of the through holes remains.
Additionally, high-density integration requires that due consideration be given to noise prevention.